It is proposed to continue the genetic analysis of the P2-P4 bacteriophage system, mainly to eludicate the regulatory mechanisms that control and couple the expression of the satellite (P4) and helper (P2) genes. Research plans include: 1) further investigations of the cox mutations of P2 and of their effects on the derepression of P2 and P4; 2) further studies concerning the role the bacterial RNA polymerase plays in the expression of the genomes of P2 and P4, employing ogr mutants of P2 and P4 (which, unlike the wild type phages, can grow in an rpoA mutant of E. coli): 3) a search for further mutants of P2 and P4 that affect derepression or other mechanism controlling gene expression (such as transactivation of P2 by P4) and the study of such mutants; and time permitting: 4) further studies of the inhibition of P4 mutants (P4 fes) by the fertility factor, F; 5) studies of P2 related phages that are "incompetent" to help P4 and studies of E. coli mutants in which P2, but P4, can grow.